In electronic devices employing electron emitters it has been a problem to maintain the integrity of the emitter surface. A major factor degrading the surface, and hence the lifetime of the device, is the return to the emitting surface of positive ions generated by the interaction of the electron beam with the residual gas in the inter-electrode spaces, and by ionic desorption from electrode and grid surfaces. These positive ions are accelerated toward the surface by the same electric field which accelerates the emitted electrons. As a consequence, the operating life of the emitter is reduced by processes such as contamination of the surface by the ion species, sputter removal of the emitting material, chemical doping of the emitting material by ion implantation, and other physical damage to the emitting surface from ion impact. The surface degradation is of particular importance in connection with negative electron affinity electron emission devices such as the junction, secondary emission devices, and photoemission devices.
Solutions suggested in the prior art have included attempts to improve the vacuum between the electrodes, introduction of methods for cleaning the electrode surface, and replenishment of the electron emitting surface as degradation occurs. It is also known to employ combined electric and magnetic fields to separate the electron and ion trajectories based on the large differences in mass between the two species. However, this technique requires the application of external magnetic fields and presents additional beam formation and deflection problems.